The present invention relates to novel compositions based on mixed oxides of cerium oxide and zirconium oxide having a high oxygen storage capacity. This invention also relates to a novel process for the preparation of the mixed oxide compositions, and the method of using the mixed oxide compositions as catalysts and/or catalyst supports, in particular, for the purification and/or conversion of exhaust gases from internal combustion engines.
Cerium oxide has been widely employed as a promoter of a catalyst for purifying exhaust gases emitted from an internal combustion engine because of its good oxygen storage ability. Typically, to improve the oxygen storage capacity (OSC), cerium oxide is usually employed as small particles having a relatively high specific surface area. Unfortunately, however, cerium oxide tends to sinter and lose surface area under high temperature conditions thereby losing their effectiveness as an oxygen storage component.
More recently, the need to thermally stabilize cerium oxide based catalysts against deactivation at higher temperatures has focused attention on doping cerium oxide with a wide range of metals oxides. To this end, numerous prior art references have proposed incorporating zirconium oxide or other oxides of rare-earth elements into cerium oxide to slow down the sintering process and provide high surface area materials. For example, Japanese Patent Application 55,15/1992, discloses a process for preparing mixed cerium oxide and zirconium oxide wherein a solution containing trivalent cerium salt and zirconium salt is co-precipitated with a base in the presence of hydrogen peroxide. The process provides mixed oxides of cerium and zirconium having a high specific surface area and an excellent heat resistance.
It has also been proposed that pure solid solutions of cerium oxide and zirconium oxide having a high surface area are required to be effective oxygen storage components in automotive catalytic converters. Various cerium oxide/zirconium oxide compositions having high surface area have been reported.
For example, U.S. Pat. No. 5,693,299 discloses cerium oxide/zirconium oxide mixed oxide having thermal stability, very high specific surface area of at least 80 m2/g. The mixed oxides are obtained by thermohydrolysis and possess a pure monophasic CeO2 cubic crystalline habit wherein zirconium is incorporated into the crystalline habit of the cerium oxide.
U.S. Pat. No. 5,607,892 also discloses cerium/zirconium mixed oxide particles having high stable specific surface area. The mixed oxides are obtained by intimately admixing a zirconium sol with a cerium sol, precipitating the mixture with a base to recover a precipitate, and thereafter calcining the recovered precipitate. An oxygen storage capacity, measured on a product calcined at 1,000xc2x0 C., of only 2.8 ml CO/g CeO2 (62.5 micromole O2 per gram of CeO2) was reported.
In order to meet stringent future emission standards, it is necessary that cerium oxide based catalysts exhibit high OSC even after exposure to temperature in excess of 1,000xc2x0 C. Since cerium based catalysts exposed to such high temperatures typically lose surface area, there is a need to develop cerium based materials which have a high OSC independent of surface area.
Further due to recent advances in engine control technology, newer engines possess a even tighter air-fuel ratio control resulting in rapid changes in oxygen partial pressure at the location of the catalyst. Catalyst useful in such engines are not only required to possess a higher oxygen storage capacity than prior known catalysts, but also a high rate of oxygen release in order to respond to such fluctuations in oxygen partial pressure. Consequently, there exists a need in the automotive industry for catalyst/catalyst support materials which possess both a high oxygen storage capacity and an increased rate of oxygen release under high temperature conditions.
Novel compositions based on mixed oxides of cerium and zirconium having an exceptionally high oxygen storage and release capacity have been developed. Mixed cerium oxide/zirconium oxide in accordance with the invention possess a nominally cubic, polyphasic crystalline habit based on a uniquely controlled domain crystalline substructure. Unexpectedly, mixed oxide compositions in accordance with the present invention possess a high oxygen storage capacity independent of surface area.
Mixed oxide compositions in accordance with the invention comprise polycrystalline particles based on cerium oxide and zirconium oxide. Crystallites comprising the polycrystalline particles are composed of regions or xe2x80x9cdomainsxe2x80x9d at the subcrystalline level having varying atomic ratios of cerium and zirconium. In accordance with the present invention, it has been found that when adjacent domains within a single crystallite sufficiently vary in their atomic ratios of cerium and zirconium, a unique crystalline sub-structure will be present which promotes increased oxygen storage and oxygen release.
Without wishing to be bound to any particular theory, it is theorized that the compositional variation between adjacent domains causes adjacent domains to possess different lattice parameters. This difference in lattice parameters is believed to result in localized strain at the domain boundaries. It is hypothesized that such localized strain along the boundaries of adjacent domains provide a network of internal pathways throughout the crystallites. It is believed that the presence of these pathways permits oxygen to be rapidly absorbed into and released from the bulk crystalline lattice, thereby providing increased oxygen storage and release capability independent of the external surface area of the particles.
Accordingly, a major advantage of the present invention is to provide novel cerium oxide/zirconium oxide compositions having a specified domain crystalline substructure which promotes an increased oxygen storage capacity and rate of oxygen release when compared to prior cerium oxide/zirconium oxide compositions.
Another advantage of the present invention is to provide novel cerium oxide/zirconium oxide compositions which exhibits a high oxygen storage capacity independent of surface area.
It is also an advantage of the present invention to provide cerium oxide/zirconium oxide compositions having a high oxygen storage capacity which compositions do not require a pure mono-phasic cubic solid solution of either cerium oxide dissolved in zirconium oxide or zirconium oxide dissolved in cerium oxide as heretofore taught by the prior art.
It is yet another advantage of the present invention to provide novel mixed oxides of cerium and zirconium which are highly effective as catalyst/catalyst support for the purification of exhaust gases.
Still another advantage of the present invention is to provide a process for the preparation and use of the novel cerium oxide/zirconium oxide compositions. Other advantages and objects of the present invention will be understood from the detail of the description, examples and claims.